Enhanced electron emissive surfaces for a thin film deposition system using ion sources

ABSTRACT

The invention pertains to the use of enhanced electron emitting surfaces to increase the supply of electrons in a thin film deposition system including the ion source in order to enhance the deposition rates of thin film materials. The use of enhanced electron emitting surfaces reduces the erosion of component parts in the ion source while increasing the rate and quality of the film deposited on the substrate. Allowing for ion source operation at lower gas pressure also increases the range of cold-cathode applications and improving operation at all pressures. The cathode section of the ion source is comprised of a reactive material that upon reaction with a reactive gas forms an insulating thin film on the cathode surface that provides an addition source of electrons for the ion beam source. Also, electron emitters located outside of the ion beam source have cathode sections that comprise enhanced electron emitting surfaces to provide electron flow to the ion beam.

RELATED APPLICATIONS

This application is a 371 of PCT/US00/20907 which claims the benefit ofthe commonly assigned provision application Ser. No. 60/146,738 filedAug. 2, 1999 the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Generally, the invention pertains to the field of vacuum thin filmposition using ion sources. Specifically, the invention focuses uponenhancing the supply of electrons in various regions of the depositionsystem including the ion source in order to enhance the deposition ofthin films onto substrates and expand the range of thin film depositionapparatus, methods, and applications.

2. Brief Description of the Prior Art

The existing literature, such as “Handbook of Ion sources” edited byBernard Wolf (CRC Press, 1995), hereby incorporated by reference,teaches that most types of ion sources as well as application apparatusrequire a source of electrons. In applications of all types of ionsources where the substrate or other surfaces being exposed to ion beamare insulating, the need exists to neutralize the electrical chargeinduced by the ion beam and this typically is done using an electronsource since electrons have negative charge. In addition, those skilledin the field of ion sources know that electron supply plays afundamental role in the ion source operation. In plasma-type ionsources, the electrons produce ions by ionizing the working gas. Theelectrons also conduct current such that power can be continuouslycoupled to the plasma. Therefore, it may be desirable to have electronemitters within the ion source to provide an enhanced continuous supplyof electrons.

A significant problem with the use of cold cathode ion sources is thatthe secondary emission processes that generate electrons in thesesystems are not very efficient necessitating the use of a high dischargevoltage to sustain stable operation. The use of high discharge voltage,however, may lead to defects in the films deposited on the substratebecause high discharge voltages may generate fast particles that maysubsequently collide with the surface of the substrate.

A significant problem with the use of hot filament thermionic electronemitters is that the operational lifetime of the emitters is verylimited, often less than 100 hours. This is especially true whenreactive gases, such as oxygen, are present in the ion source.Similarly, a problem with dispenser type thermionic emitters is that theoperating life of these units is also limited, and compatibility withreactive gases remains a problem even though such emitters operate atlower temperature and produce higher current as compared to thermionicelectron emitters. Similarly, hollow cathode electron emitters have alifetime of only about 1000 hours as disclosed by U.S. Pat. Nos.3,156,090; 3,913,320; 3,952,228; 3,956,666; and 3,969,646, each herebyincorporated by reference.

Another problem with existing sputtering techniques is the erosion ofthe component parts of the ion source due to incidental sputtering ofmaterial from the surface of such component parts. This incidentalsputtering reduces the operating life of such component parts and alsoincreases the costs of maintenance and of operating the ion source.

Still another problem associated with incidental sputtering of materialfrom surfaces of component parts is that the material sputtered fromthese surfaces may be deposited onto the substrate surface contaminatingthe material deposited on the substrate or it may create flaws in thematerials deposited on the substrate surface.

The present invention discloses a system that addresses each of theabove-mentioned problems associated with the existing methods andapparatus used for deposition of films onto the surfaces of substrates.The invention while alleviating the above-mentioned problems does notreduce the existing range of ion source applications. On the contrary,it expands the cold-cathode ion source application range by allowing forsource operation at lower gas pressure and improving operation at allpressures. Surprisingly, this invention makes use of a phenomenon knownas “target poisoning” which prior to the instant invention has beenconsidered a serious problem in the processing of substrates using coldcathode magnetron sputtering. Target poisoning occurs when sputteringmetal or semiconductor material from a magnetron target in the presenceof a reactive gas, such as oxygen. Under these conditions, the targetsurface tends to develop and sustain a thin ceramic-like insulatinglayer of oxide that in most cases is the same material that is beingdeposited on the substrate. This “target poisoning” results in a lowerremoval rate of metal from the target and a correspondingly lowerdeposition rate on the substrate because the ceramic-like insulator ismuch harder to sputter than the bulk target material. As such the priorart, teaches that the deposition of such ceramic-like insulator shouldbe avoided in magnetron systems by the use of sputtering methods havinghigh removal rates of material from the target surface thereby insuringthat oxide will not accumulate. One example of such a method is taughtby M. Alex, C. Van Nutt and S. Gupta in ADC-Reactive Sputtering ofAl₂O₃″ attached hereto and hereby incorporated by reference. Efforts toeliminate “target poisoning” acted to teach away from the direction theinstant invention takes in that by focusing on methods to eliminatepoisoned surfaces from magnetron sputtering processes it divertedattention away from the beneficial uses of “poisoned materials”.

SUMMARY OF THE INVENTION

The present invention includes a variety of aspects that relate to theprocess of film deposition onto substrate material using ion sources.The invention addresses the efficiency, the throughput of the depositionprocess, the range of ion source operating parameters, and the qualityof thin film deposition onto substrates.

Accordingly, a significant object of the invention is to produce astable and consistent supply of electrons over a wide range of dischargeparameters. Attaining this goal can be difficult because, as mentionedabove, the secondary electron emissions produced by the cold-cathodeprocessing can be low. This invention focuses on increasing the electronemissive properties of various surfaces within and outside of the ionsource thereby providing an enhanced supply of electrons.

Another object of the invention is to increase the output of the ionbeam of ion sources that do not utilize hot cathodes. When the output ofthe ion beam is increased, the deposition rate onto the substrate isincreased. This increased rate is particularly desirable for thecommercial processing of substrates where increased throughputtranslates into lower cost of production. Increasing the output of theion beam, however, requires an additional supply of electrons tointensify the discharge inside the ion source and to negate theadditional charges generated. The instant invention increases the supplyof electrons that in turn allows the use of increased ion beam output.

Still another object of the invention is to reduce the erosion of theion beam apparatus elements. The erosion of these apparatus elements notonly shortens the operating life of the element itself but may also bedeposited as a contaminate on the substrates which are being processed.This invention reduces the sputtering of materials from the apparatuselements helping to alleviate both of these problems.

Yet another broad goal of the invention is to provide surfaces withenhanced electron emissive properties for industrial applications thatare inexpensive with respect to the initial investment for the enhancedelectron emitting surfaces and also inexpensive with respect tomaintenance of such surfaces.

Naturally further objects of the invention are disclosed throughoutother areas of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view that illustrates a particular embodiment ofthe invention wherein the cathodes of an ion source contain a materialthat may grow insulating films in the presence of a reactive gas;

FIG. 2 is a perspective view of the embodiment of the invention shown inFIG. 1 in which an ion beam is applied to a substrate material;

FIG. 3 illustrates an embodiment of the invention showing amagnetron-like device with a poisoned cathode used to neutralize chargesin an ion beam and to supply electrons to enhance the source operation;

FIG. 4 illustrates an embodiment of the invention shown in FIG. 3showing a magnetron-like device which has an annular shape surroundingthe ion source; and

FIG. 5 illustrates an embodiment of the invention showing the electronemitting device with poisoned surfaces inside a Penning type ion source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention includes a variety of aspects that may be combinedin different ways. Each of these aspects is discussed separately below.

As discussed earlier, target poisoning significantly reduces the amountof material that can be sputtered from the target material. This maybecome a benefit for the ion source parts because sputtering of the ionsource material is undesirable as discussed previously. “Poisoned”surfaces also have enhanced electron emission properties that are verystable and consistent over wide range of discharge parameters. In filmdeposition methods, using cold cathode ion beam sources in which thesupply of electrons can be deficient, such poisoned or coated surfacescan be beneficial when they are properly used within the depositionapparatus.

In one embodiment of the invention shown by FIG. 1 there is illustrateda cross section of an ion source that incorporates the principles ofthis invention. An ion source 10 that is generally based on an anodelayer design is generally comprised of a grounded steel housing 12 thatincludes a gas containment housing 14. The gas containment housing 14contains a magnetic field generating means 16 that may be a permanentmagnet or electromagnetic coil. A positively biased anode shown in crosssection at 18 and 20 is spaced a short distance λ below openings ormagnetic gaps 22 and 24. The surfaces such as 26 that form the magneticgaps 22 and 24 in the housing 12 are the cathodes of the ion source 10.The openings or magnetic gaps 22 and 24 allow the ion beam generated toexit the ion source 10. To enhance the generation of electrons in theion source the cathodes 26 may be comprised of a material such asaluminum, titanium, silicon or other material that in the presence of areactive gas forms an insulating thin film. As an alternate, thecathodes may be formed by applying a coating 27 to the surfaces of thecathodes 26 with these materials. These cathodes represent the “poisonedsurfaces” as described herein above. A reactive gas 28, such as oxygen,on the order of about five per cent to one hundred per cent where theapplication permits, is introduced into the manifold via the gas feeds30 and 32. The reactive gas 28 on the cathodes 26 will cause thesesurfaces to produce an enhanced supply of electrons.

FIG. 1 is an example of an embodiment of the invention of an ion sourcewith a cold cathode closed drift path. This type of ion source issimilar to the type manufactured by Advanced Energy Industries, theassignee of the instant application, although the invention may be usedwith any similar ion source. With the modification to thecathode-surfaces 26 as described above, the minimum gas pressurerequired for operation of the cold cathode closed drift ion source wasreduced down to about 1×10⁻⁶ Torr. The capability of operating at thislower pressure expands the range of applications for cold-cathode ionsources. At higher pressures the ion beam current was significantlyincreased as compared to unmodified or uncoated cathodes under the sameoperating conditions.

Referring to FIG. 2 there is illustrated a perspective view of the ionsource 10 that was shown in cross section in FIG. 1. As seen in thisFigure the magnetic gaps 22 and 24 through which the ions are producedis in the form of a racetrack, or oval configuration. The ion source 10is operated in a diffused beam mode and uses a working gas 28 of argonwith about five per cent or more oxygen, although other reactive gasesmay be used as described herein above. A glass substrate 34 exposed toan ion beam 36 produced by an ion source 10 with an aluminum or aluminumcoated cathodes 26 had a reduced amount of contamination material on thesubstrate, and the contamination material was transparent since thedeposited aluminum was oxidized. Under the same conditions a similarglass substrate processed with traditional or uncoated cathodes washeavily contaminated with iron to the degree that it was nottransparent. Similar reductions in contamination of the substratecombined with making the contamination less harmful (e.g. transparent)may be expected in other similar applications.

In another embodiment of the invention, as shown in FIG. 3, the“poisoned” surfaces may be implemented as a little Penning cell ormagnetron like electron emitter device 38 located outside of the ionsource 40 in close proximity to the ion beam 42. The electron emitterdevice shown in FIG. 3 is in cross section, but such a device may alsohave an annular shape circumventing the ion source as shown by FIG. 4.In either case, the cathode 44 can be made out of aluminum or isaluminum coated and oxygen gas can be fed into the device via gasdelivery device 46. The power supply 48 can be of DC, AC or pulsed DCtype, depending on the arc suppression needs in the particularapplication. The positive terminal of the power supply is shown in FIG.3 connected to a ground, but it can be also connected to any electrodeof the ion source or additional electrodes in the apparatus, dependingon the particular application needs. The Penning cell or magnetron-likedevice 38 may also have an optional gas containing case 50 around it toreduce gas flow into the system and also an optional discharge ignitionelectrode 52.

FIG. 4 illustrates a perspective view of the enhanced electron beam 54generated by the annular shaped electron emitter 40 around the ion beam42. In FIGS. 3 and 4 the ion source 40 may be multi-cell ion sourcesimilar to the type manufactured by Advanced Energy Industries, theassignee of the instant application, although other ion sources may beused.

In yet another embodiment the magnetron-like electron emitter device 56similar to that described in previous paragraph can be installed insideof the ion source 58, as shown in FIG. 5. Here the ion source is of thePenning type multi-channel source and the construction could be similarto one described by A. A. Bizioukov et. al. in “Multichannel source ofsynthesized ion-electron flow”, Rev. Sci. Instrum, 67 (12), 1996, page4117-4119, which is hereby incorporated by reference, but could takevarious other configurations as well. With regard to this embodiment,the anode 60 is conical and the separately powered magnetron-likeelectron emitter device 56 is mounted at the inner end of the anode. Thedevice may also have an ignition electrode 62. This electron emittingdevice may use the same magnetic field as the ion source which iscreated using a solenoid or other magnetic field generating device 64.

This embodiment also uses a cathode that has a surface comprised of amaterial such as aluminum, titanium, silicon or other material that inthe presence of a reactive gas forms an insulating thin film. Ignitingthe reactive gas with the cathode reactive materials generates anelectron flow that combines with the ion beam. The “poisoning” of thecathode with the insulating thin film serves to provide an additionalsource of electrons for the ion beam.

As can be easily understood, the basic concepts of the present inventionmay be embodied in a variety of ways. It involves both techniques forcreating and using enhanced electron emissive surfaces as well ascomponents that are specifically configured for various sputteringapplications. In this application, the techniques for creating and usingenhanced electron emissive surfaces are disclosed as part of the resultsshown to be achieved by the various devices described and as steps whichare inherent to utilization. They are simply the natural result ofutilizing the devices as intended and described. In addition, while somedevices are disclosed, it would be understood that these not onlyaccomplish certain methods but also can be varied in a number of ways.Importantly, as to all of the foregoing, all of these facets should beunderstood to be encompassed by this disclosure.

Where the invention is described in device-oriented terminology, eachelement of the device implicitly performs a function. Apparatus claimsmay not only be included for the device described, but also method orprocess claims may be included to address the functions the inventionand each element performs. Neither the description nor the terminologyis intended to limit the scope of the claims.

It should be understood that a variety of changes may be made withoutdeparting from the essence of the invention. Such changes are alsoimplicitly included in the description.

In addition, each of the various elements of the invention and claimsmay also be achieved in a variety of manners. This disclosure should beunderstood to encompass each such variation, be it a variation of anembodiment of any apparatus embodiment, a method or process embodiment,or even merely a variation of any element of these. Particularly, itshould be understood that as the disclosure relates to elements of theinvention, the words for each element may be expressed by equivalentapparatus terms or method terms—even if only the function or result isthe same. Such equivalent, broader, or even more generic terms should beconsidered to be encompassed in the description of each element oraction. Such terms can be substituted where desired to make explicit theimplicitly broad coverage to which this invention is entitled. As butone example, it should be understood that all action might be expressedas a means for taking that action or as an element that causes thataction. Similarly, each physical element disclosed should be understoodto encompass a disclosure of the action that that physical elementfacilitates. Regarding this last aspect, the disclosure of an “electronemitter” should be understood to encompass disclosure of the act of“emitting electrons” whether explicitly discussed or not and,conversely, were there only disclosure of the act of “emittingelectrons”, such a disclosure should be understood to encompassdisclosure of an “electron emitter”. Such changes and alternative termsare to be understood to be explicitly included in the description.

Any references mentioned in this application for patent as well as allreferences listed in any information disclosure filed with theapplication are hereby incorporated by reference or should be consideredas additional text or as an additional exhibit or attachment to thisapplication; however, to the extent statements might be consideredinconsistent with the patenting of this/these invention(s) suchstatements are expressly not to be considered as made by theapplicant(s).

What we claim is:
 1. An ion beam source, comprising: a metallic housing;a gas input into the housing for receiving a reactive gas; a magneticfield generating means for producing a magnetic field within thehousing; at least one magnetic gap wherein the opposite sides of themagnetic gap define a cathode attached to the housing; at least oneanode in the housing in proximity to the magnetic gap; and the cathodesection of the housing comprising a reactive material wherein reactionof the reactive gas with the cathode reactive material surface forms aninsulating thin film that enhances electron emission.
 2. An ion beamsource as recited in claim 1 wherein the cathode reactive material isfrom the group of materials such as aluminum, titanium, silicon, ormaterials that when reacting with reactive gases forms an insulatingthin film.
 3. An ion beam source as recited in claim 2 wherein thereactive gas is from the group of gases such as oxygen, fluorides, orother gases and gas mixtures that when reacting with the cathodereactive material surface forms an insulating thin film.
 4. An ion beamsource as recited in claim 1 wherein the parts of the cathode surfacesare coated with a reactive material wherein reaction of the reactive gaswith the cathode reactive material surface forms an insulating thinfilm.
 5. An ion beam source as recited in claim 4 wherein the cathodereactive material is from the group of materials such as aluminum,titanium, silicon, or materials that when reacting with reactive gasesform an insulating thin film.
 6. An ion beam source as recited in claim5 wherein the reactive gas is from the group of gases such as oxygen,fluorides, or other gases and gas mixtures that when reacting with thecathode reactive material surface forms an insulating thin film.
 7. Anion beam source in combination with an electron emitter, comprising: asource for generating an ion beam; an electron emitter in proximity tothe ion beam generally comprising: a magnetic field generating means forproducing a magnetic field; a cathode comprising a reactive materialsurface wherein reaction of the reactive gas with the cathode reactivematerial surface forms an insulating thin film that enhances electronemission; and a power supply connected to the cathode section of thehousing for generating an electron flow for interaction with the ionbeam.
 8. An ion beam source in combination with an electron emitter asrecited in claim 7 wherein the cathode reactive material is from thegroup of materials such as aluminum, titanium, silicon, or materialsthat when reacting with reactive gases form a thin insulating film. 9.An ion beam source in combination with an electron emitter as recited inclaim 8 wherein the reactive gas is from the group of gases such asoxygen, fluorides, or other gases and gas mixtures that when reactingwith the cathode reactive material forms a thin insulating film.
 10. Anion beam source in combination with an electron emitter as recited inclaim 7 wherein the cathode section of the housing is coated with areactive material wherein reaction of the reactive gas with the cathodereactive material forms an insulating thin film.
 11. An ion beam sourcein combination with an electron emitter as recited in claim 10 whereinthe cathode reactive material is from the group of materials such asaluminum, titanium, silicon, or materials that when reacting withreactive gases forms an insulating thin film.
 12. An ion beam source incombination with an electron emitter as recited in claim 11 wherein thereactive gas is from the group of gases such as oxygen, fluorides, orother gases and gas mixtures that when reacting with the cathodereactive material forms an insulating thin film.